Detection of metallicity correlations in 100 nearby galaxies

Abstract

In this paper, we analyse the statistics of the 2D gas-phase oxygen abundance distributions of 100 nearby galaxies drawn from the Calar Alto Legacy Integral Field spectroscopy Area survey. After removing the large-scale radial metallicity gradient, we compute the two-point correlation functions of the resulting metallicity fluctuation maps. We detect correlations in the majority of our targets, which we show are significantly in excess of what is expected due to beam-smearing, and are robust against the choice of metallicity diagnostic. We show that the correlation functions are generally well-fit by the predictions of a simple model for stochastic metal injection coupled with diffusion, and from the model we show that, after accounting for the effects of both beam smearing and noise, the galaxies in our sample have characteristic correlation lengths of ∼1 kpc. Correlation lengths increase with both stellar mass and star formation rate, but show no significant variation with Hubble type, barredness, or merging state. We also find no evidence for a theoretically predicted relationship between gas velocity dispersion and correlation length, though this may be due to the small dynamic range in gas velocity dispersion across our sample. Our results suggest that measurements of 2D metallicity correlation functions can be a powerful tool for studying galaxy evolution.

A geostatistical analysis of multiscale metallicity variations in galaxies [I]: Introduction and comparison of high-resolution metallicity maps to an analytic metal transport model

“…The predicted metallicity correlation structure of the KT18 model has been compared to IFS data in previous works (Kreckel et al 2020;Li et al 2021), albeit using different methodologies. In this work and in Kreckel et al (2020), the metallicity gradient is fit using a least-squared method and subtracted, whereas in Li et al (2021) radial trends are re-moved by subtracting the median metallicity in 0.2 kpc wide annuli. Their approach has the advantage of better capturing non-linear metallicity trends, at the cost of introducing discontinuities in the residual metallicity field at the boundaries between annuli.…”

“…While Li et al (2021) fit a two-point correlation function to the residual metallicity fluctuation map, in this work, a semivariogram is computed. This approach affords two advantages over the method of Li et al (2021): firstly, with the semivariogram, it is easier to correct for the variance caused by measurement error (in Li et al 2021, the effects of measurement error complicate the correlation function, adding two new parameters to the KT18 model). Secondly, the semivariogram is sensitive to the magnitude of metallicity fluctuations, whereas the two-point correlation function is not.…”

“…This additional degree of freedom revealed a tension between the model and the data that the two-point correlation functions could not identify. Finally, in this work, the values of the parameters of the KT18 model are computed a priori from global galaxy properties, whereas in Li et al (2021), the parameters are fit to best reproduce the observed two-point correlation function for each galaxy. Because of this, some computed parameters of best fit are inconsistent with the physical properties of local galaxies, such as star-formation timescales of 1 Gyr.…”

“…Using a sample of 100 local galaxies from the CALIFA survey, Li et al (2021) found a weak positive correlation (ρ = 0.36 ± 0.07) between the metallicity mixing scale and the galaxy size, measured by R25. The fact that we do not recover this correlation in our data is likely due to the small number of galaxies analysed in this work.…”

“…Using a different methodology, by computing the mean standard deviation around larger and larger sub-regions of PHANGS galaxies, Kreckel et al (2020) found that metallicities of Hii regions stopped being correlated after ∼ 600 pc. By computing the two-point correlation function in metallicity fluctuations after subtracting a radial trend and fitting an analytical model to 100 galaxy maps measured by the CALIFA survey using Markov Chain Monte-Carlo methods, Li et al (2021) determined the metallicity correlation length to be ∼ 1 kpc. These contrasting results come from vastly different methodologies, each with their own set of implicit assumptions about the metallicity structure of galaxies.…”